Mud microbes power turtle-tracking sensors

Ocean sensors that feed on bacteria living in deep sea sediment are being developed by US researchers. The microbial fuel cells used to power the sensors are already helping to remotely track green turtles in the wild.

Sophisticated sensors let oceanographers study various aspects of the ocean over long periods of time, but replacing these sensors' batteries can be a complex and costly process, especially if they are positioned far out at sea and deep beneath the surface.

Oceanographers at Oregon State University, US, say a simple solution is to harness microbes in sea-floor sediment to provide a constant supply of power instead.

So they have developed fuel cells that feed on microbes - using an electrode to collect electrons released by bacteria living in the low oxygen environment. These electrons are diverted around a circuit to provide power and are then combined with oxygen and hydrogen ions to form water, balancing the electrochemical reaction.

Sewage power

Microbes used in this way are have previously been sourced in sewage, but Mark Nielsen and colleagues at Oregon State University are working on systems that use those found in ocean sediment instead. They have now begun testing prototypes in ocean environments that provide sufficient power to run simple oceanographic sensors.

"Sea floor sediment is a microbe-rich anoxic environment that can provide electrons," Nielsen told New Scientist.

The fuel cells developed so far feature an open bottomed chamber that sits over a section of sediment. This chamber becomes colonised by the same microbes living beneath the surface of the sediment. "You could say it becomes an extension of that environment," Nielsen explains. "We can use an electrode with very high surface area inside the chamber to get maximise power."

Power output drops off, however, when the available energy from microbes in an area has been used up. Nielsen and colleagues are trialling two ways to deal with this. One is to have two chambers and to alternate between them, giving the microbes in the inactive one a chance to refresh the chemical process needed to generate power.

Military hopes

This design is already being used in sensors that are listening for acoustically tagged turtles in a US Navy project in San Diego Bay, California, US. The navy hopes a similar system could listen for ships or submarines too.

Another design being tested in Monterey Bay, California, has a chamber with a valve at the top that lets water seep up through the main chamber, providing a constant flow of water and refreshing the microbes' food source.

That boosted power output by around five times. "It produces about 60 milliwatts of power, enough for some low powered sensors," says Nielsen.

The team are now working on using a similar design to power the acoustic homing beacons oceanographers use to mark ocean floor sites they want to visit again.

The batteries normally used to power oceanographic sensors typically last around a year. So far, the microbial fuel cells have operated for over 200 days, with roughly constant power output, says Nielsen.

For the time being, the fuel cells are no cheaper than using batteries. "We can roughly match batteries now," says Neilsen. "If we make more improvements, they will be cheaper by saving money on ships and other costs."

"Most microbial fuel cell work is done at small scale in the lab," says Jung Rae Kim, who works on powering them with wastewater at Glamorgan University, UK. Testing larger cells at depth is a good idea, Kim adds.

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